40 OF THE ASSIMILATION OF CARBON. 



than that yielded by lands which are not manured. 

 The discussion as to the manner in which manure 

 acts has nothing to do with the present question, 

 which is, the origin of the carbon. The carbon must 

 be derived from other sources ; and as the soil does 

 not yield it, it can only be extracted from the atmo- 

 sphere. 



In attempting to explain the origin of carbon in 

 plants, it has never been considered that the ques- 

 tion is intimately connected with that of the origin 

 of humus. It is universally admitted that humus 

 arises from the decay of plants. No primitive 

 humus, therefore, can have existed, — for plants must 

 have preceded the humus. 



Now, whence did the first vegetables derive their 

 carbon ? and in what form is the carbon contained 

 in the atmosphere ? 



These two questions involve the consideration of 

 twt) most remarkable natural phenomena, which by 

 their reciprocal and uninterrupted influence maintain 

 the life of the individual animals and vegetables, 

 and the continued existence of both kingdoms of 

 organic nature. 



One of these questions is connected with the inva- 

 riable condition of the air with respect to oxygen. 

 One hundred volumes of air have been found, at 

 every period and in every climate, to contain 21 

 volumes of oxygen, with such small deviations that 

 they must be ascribed to errors of observation. 



Although the absolute quantity of oxygen con- 

 tained in the atmosphere appears very great when 

 represented by numbers, yet it is not inexhaustible. 

 One man consumes by respiration 25 cubic feet of 

 oxygen in 24 hours ; 10 cwt. of charcoal consume 

 32,066 cubic feet of oxygen during its combustion ; 

 and a small town like Giessen (with about 7000 

 inhabitants) extracts yearly from the air, by the 

 wood employed as fuel, more than 551 millions of 

 cubic feet of this gas. 



When we consider facts such as these, our former 



